The present invention relates to a method of preparing encapsulated pigments comprising a chromophoric [color-bearing] substance which is encapsulated in a transparent casing formed of a substance stable against chemical attacks and vitrification by means of compounding the chromophoric substance or its initial components with the encapsulating substance or its initial components and by a subsequent calcination at 500 to 1200.degree. C, optionally in the presence of a mineralizer.
Encapsulated pigments are coloring bodies in which discrete, inorganic color particles are embedded on all sides in mostly crystalline, transparent encasing substances, whereby the encasing substances are stable against vitrification and other chemical attacks. In principle, all known chromophoric, temperature-stable substances can be encapsulated, in particular cadmium sulfoselenides or sulfides, or sulfides, selenides and sulfoselenides of mercury and zinc, as well as gold purple, iron oxide and other colored metal oxides and metal oxide mixtures. Any suitable mixtures can also be used as is known in the art. In particular, zirconium silicate, zirconium oxide, silicon oxide, tin oxide, aluminum oxide, various spinels, zinc silicate, zirconium phosphate and aluminum phosphate have proven to be stable encasing substances. These encasing substances resist the attack of glass flows, oxygen at high temperatures and chemical substances such as e.g. acids.
DE-OS 23 12 535 describes a method of preparing such inclusion pigments in which a mixture of the chromophoric substance or compounds from which the colored substance is formed and of the encasing substance or compounds from which the encapsulating substance forms during annealing is annealed at 800.degree. C to 1200.degree. C, optionally in the presence of a mineralizer such as e.g. lithium fluoride, magnesium fluoride and similar substances. These pigments are heated and cooked with acids in order to remove still-adhering, acid-soluble substances. It is also known that the chromophoric component and the encapsulating substance or its initial compounds can be precipitated from an aqueous solution in common or in series and subsequently calcined (e.g. EP 0,074,779; DE-OS 33 45 413).
All previously known methods do not yield optimum coloring bodies as regards their color intensity; low yields of inclusion pigments and too-high a portion of relatively large particles are obtained.